1. Field of Invention
The invention relates generally to stencil cleaning methods and apparatus, and more particularly to a stencil printer having at least one web-material roller.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a circuit board having a pattern of pads or some other conductive surface onto which solder paste may be deposited. The circuit board is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, may be used to properly align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. Once a circuit board has been properly aligned with the stencil in the printer, the circuit board may be raised to the stencil (or the stencil may be lowered to the circuit board), solder paste may be dispensed onto the stencil from a standard solder paste cartridge, and a wiper blade or squeegee may traverse the stencil to force the solder paste through apertures formed in the stencil and thereby onto the circuit board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain desired viscosity to facilitate filling of the apertures in the screen or stencil.
In some prior art stencil printers, any excess solder paste remaining under the squeegee after it has fully traversed the stencil, remains on the stencil when the squeegee is returned to its initial position for printing on a subsequent circuit board. In some instances, as the squeegee passes the solder paste over the apertures when returning to its initial position, minute amounts of solder paste may seep through the apertures to accumulate at the bottom side of the stencil. This presents various problems such as the solder paste being inadvertently disposed on the unintended areas of the circuit boards. Also, as the solder paste hardens, it complicates the alignment procedure of a circuit board with the stencil. Also, such hardened solder paste may jeopardize the quality of subsequent printing operations. Therefore, it is highly desirable to remove the excess solder paste that forms on the bottom of the stencil.
U.S. Pat. No. 5,918,544 to Doyle represents one prior art stencil printer having a well-known method and apparatus for cleaning the bottom of the stencil. Doyle discloses a wiping system that is positioned near the vicinity of the stencil and moves beneath the stencil from one end of the stencil to the other end. As the stencil wiper system moves beneath the stencil, it wipes off excess solder paste at the bottom of the stencil.
FIGS. 1 and 2 disclose such a prior art stencil wiper system, generally indicated at 100. As shown, a paper supply of a prior art stencil wiper system 100 includes a roll of paper 101 housed on a supply roller 103, several paper guide rollers 201, a take-up roller 105 for receiving used paper, and a paper driver 107 for moving the paper across the stencil in a linear direction from the supply roller 103 to the take-up roller 105. In one embodiment, the stencil wiper system 100 further includes a hollow solvent tube 109 with numerous small openings formed along the length of the tube 109, and a wiper blade 203 having a vacuum plenum 205 for removing excess moisture and hardened solder paste from the paper as it travels underneath the stencil. The wiper blade 203 moves the web between a position in which the paper is spaced away from the stencil and a position in which the paper engages the stencil. During a cleaning operation, a paper winder motor of the paper driver 107 rotates the take-up roller 105 (and paper supply roller 103), which passes paper through the paper guide rollers 201. Between the paper guide rollers 201 there is the hollow solvent tube 109 that is filled with solvent by a solvent pump, which causes the solvent tube 109 to squirt solvent through its numerous holes onto the paper as it passes the solvent tube 109. The solvent impregnated paper is passed over the wiper blade 203, which holds the paper in place as the stencil moves over the paper, thereby cleaning the stencil.
Changing paper rolls and cores for disposal of used paper can be a difficult process. In particular, paper is typically supplied wrapped around a cardboard core (e.g., a tube) that is placed around the supply roller 103. After the paper is used, the paper is wrapped around another cardboard core engaged to the take-up roller 105. To provide a stable supply of paper without damaging the core or the paper, the cores (i.e., the core of the supply roll and the core on the take-up roller) may be sized to fit firmly around the respective roller. A single roller may have difficulty securely engaging cores having slightly different sizes due to manufacturing differences.